As a broadband era has been developed, introduction of a WDM (Wavelength Division Multiplexing) transmission system, which can communicate using plural optical wavelengths, has proceeded. Recently, application of a DWDM (Dense Wavelength Division Multiplexing) device, which can achieve a further high-rate transmission by multiplexing several tens of the optical wavelengths, has spread.
Accordingly, in the WDM transmission system, light sources corresponding to each of the wavelengths are required, and number of the required light source has been dramatically increasing according to high-multiplexing. Further, recently, a research for commercializing a ROADAM (Reconfigurable Optical Add/Drop Multiplexers) in which an arbitrary wavelength is added/dropped at each node has proceeded. Transmission capacity by wavelength multiplexing can be expanded and, further, light path switching can be achieved by changing the wavelength in the case of introducing the ROADM system. Thus, degree of freedom of an optical network is extremely increased.
A distributed feedback laser diode (Hereinafter, referred to as a DFB-LD) oscillating in a single axial mode has been used as the light source for the WDM transmission system from the view point of usability and high-reliability thereof in the past. A diffractive grating of about 30 nm depth is formed on whole area of a resonator in the DFB-LD, and it is possible to obtain a stable single-axial-mode oscillation the wavelength of which corresponds to a product of a period of the diffractive grating and twice of an equivalent refractive index.
However, the DFB-LD cannot tune in an extensive oscillating wavelength range while the DFB-LD can obtain the stable single-axial-mode oscillation. Accordingly, the DFB-LDs that are different in only the wavelength for each of ITU (International Telecommunication Union) grids to configure the WDM transmission system using the DFB-LD. That is, the different DFB-LD needs to be used for each wavelength, and thus, shelf control cost increases and excessive inventory for corresponding malfunction needs to be kept. Further, in the ROADM in which the light path is changed by the wavelength, a wavelength-tunable width is limited to about 3 nm that is a wavelength range changeable by temperature change when a general DFB-LD is used. As a result, it is difficult to configure the optical network utilizing an advantage of the ROADM that positively uses wavelength resource.
A research for the tunable laser has been done for overcoming such problem included in the existent DFB-LD and achieving the single axis-mode oscillation in a wide wavelength range. The tunable laser is generally categorized into two types including a type in which a wavelength-tunable structure and a laser resonator are introduced into the same device, and a type in which the wavelength-tunable structure is introduced outside the device.
In the former tunable laser in which the wavelength-tunable structure and the laser resonator are introduced into the same device, a light-emitting area and a distributed-reflection area are separately provided in the same device. For example, a DBR-LD (Distributed Bragg Reflector Laser Diode) is known as this type of the tunable laser. Further, sampled-grating-DBR-LD and a SSG (Super Structure Grating)-DBR-LD in which light-emitting area is sandwiched with diffractive gratings the diffractive grating periods of which are periodically changed in the DBR-LD are known. Although a wavelength-tunable range of the DBR-LD is limited to about 10 nm at a maximum, in the sampled-grating-DBR-LD, a wavelength-tunable operation over 100 nm and a semi-continuous wavelength-tunable operation at 40 nm are achieved by adeptly using the vernier effect that is characteristics in the present structure.
A system in which a diffractive grating is provided outside a device and a wavelength-tunable operation is done by precisely adjusting an angle and distance of the diffractive grating is proposed as the latter tunable laser in which the wavelength-tunable structure is introduced outside the device.
Further, as another example, a structure in which an optical resonator consists of a PLC (Planar Lightwave Circuit) on which an external resonator is formed and a LD (Laser Diode) or SOA (Semiconductor Optical Amplifier) is directly mounted on the PLC is proposed (e.g., PTL1). In the tunable laser, a step is provided for mounting the SOA, for example, on the PLC. The SOA is set in this step part. Thus, an active layer of the SOA and a waveguide of the PLC are in a position relationship capable of being aligned.